The handsome scion of a Manhattan hotel conglomerate was found dead of a suspected drug overdose in his posh Union Square pad, law-enforcement sources said Monday.

Charlie Denihan — a 29-year-old Ivy League grad whose father runs the Denihan Hospitality Group — was discovered by a roommate around 7:30 p.m. Sunday inside their apartment at 15 Union Square West, sources said.

Cops found crystal meth, cocaine and the club drug GHB in the home, sources said. No criminality or foul play is suspected, they said.

But Denihan’s death was the third suspected drug fatality in the 13th Precinct within four hours Sunday — and GHB was found at each scene, sources said.

The other two victims were men who’d been partying at a bar and then went back to the West 19th Street home of one of them, where the drug was found, sources said. Their bodies were discovered around 3:30 p.m.

There is no indication that Denihan and the men knew each other. But authorities are having the GHB from both apartments tested to see if it came from the same batch and may have been tainted, sources said.

In an eerie twist, Denihan posted a photo of himself on Instagram on Halloween — waving from a coffin.

One of the mottos on his account says, “I’m a bad influence but … damn I’m fun!!!”

His last posting, March 12, appeared to be a photo of a kite that he’d just flown into a tree.

A woman answering the phone at the family’s home Monday said, “It’s not a very good situation,’’ before starting to cry and hanging up.

The stricken family released a statement saying:

“We are devastated at the tragic and sudden loss of Charlie. He was a loving and remarkable son, brother, nephew, cousin, Godfather and friend. In this tragic time, we ask that you remember him in your prayers and respect our privacy as we mourn his passing.”

Denihan’s father, Patrick, runs the family business with his sister, Brooke Barrett.

The firm’s holdings include The James and Affinia Hotels, as well as the tony Surrey, The Mansfield and Shoreham in New York City.

Charlie Denihan grew up in privilege, attending St. David’s Catholic school, where annual tuition is about $40,000, Deerfield Academy and Cornell University.

He was a management trainee at his family’s firm, as well as co-founder of the company Silverlake to Soho, which helps promote musicians, according to his LinkedIn page.

Charlie Denihan was frequently photographed at society events and dated Jules Kirby, who has been described in gossip columns as “New York’s No. 1 bachelorette” and appeared on Tinsley Mortimer’s reality show “High Society.’’